The present invention relates in general to vapor/particle detection devices, and more particularly concerns novel apparatus and methods for quickly, easily, and inexpensively calibrating vapor/particle detection devices. Examples of vapor/particle detection devices are described in U.S. Pat. No. 5,092,217 to Achter et al., entitled "Method of Calibrating a Vapor Detector," issued on Mar. 3, 1992, assigned to the assignee of the present invention, and incorporated herein by reference in its entirety.
Increasing interest in detecting the presence of contraband substances, such as explosives, controlled narcotics, perfumes, interferents, and numerous other materials, has led many organizations worldwide, most notably airports, to purchase vapor/particle detection devices. These detectors, which are able to collect, analyze, and identify trace amounts of substances entrained in an airstream, are generally of either of two types. The first type is a hand-held "gun" detector, a device sufficiently portable to be carried to the object or person to be tested. The second type is a stationary "portal" detector, a chamber or booth within which objects or persons are tested.
For vapor/particle detectors of either type to remain effective, the device must be periodically calibrated and adjusted or otherwise serviced as necessary. Calibration can involve either a general, "front-to-back" qualitative assessment of whether the detector properly indicates the presence of contraband, or a particularized determination of the actual, quantitative sensitivity of the detector.
Frequent calibrations can be of particular importance in many field applications of vapor/particle detectors. In the field, generally only a small fraction of the sampled items contain contraband substances, making positive indications a rare occurrence. Thus, because the continued absence of positive indications is unremarkable, frequent calibration serves two important purposes. First, it determines whether the threshold of detection has drifted unacceptably. Second, it keeps the operator familiar with the general operation of the equipment, and how it responds to the presence of contraband.
The need frequently to calibrate vapor/particle detectors can however be complicated by the fact that the system operator typically has no or only moderate technical training. A cumbersome and difficult calibration procedure is thus more likely to be mishandled or foregone completely. Similarly, if the necessary calibration equipment is not sufficiently portable to be included within, or brought to, the portal, the difficulty and expense of calibration increases.
The calibration of portal detectors poses further difficulties as well. In particular, because the airflow characteristics through a portal are rarely uniform, certain regions of the portal will be more sensitive than others. A calibration technique or apparatus that is unable to isolate and quantify these spatial variations in sensitivity therefore provides a less-complete picture of overall detector performance.
Determining detector sensitivity quickly, easily, and accurately is important also during the design and testing of vapor/particle detection devices. As with field testing, it is of course important to evaluate the qualitative front-to-back performance of the detector periodically under a variety of conditions. Quantitative sensitivity evaluation is also of particular value to the detector designer. Generally, vapor/particle detection devices include a "sample collector train," which collects and concentrates vapor and/or particles entrained in a stream of sample air. Because these collector trains have complex heat transfer and air flow properties, slight changes in their design can lead to dramatic shifts in performance. In the absence of a calibration technique and/or apparatus that allows the sensitivity of the detector to be accurately evaluated, detector designers may be unable to determine the impact of a given design change on collector efficiently and (in the case of portal detectors) spacial sensitivity.
One calibration technique is to employ actual contraband, such as narcotics or explosives, in a setting that reflects real world conditions (e.g., on a person or in luggage). Field operators, however, are generally untrained in the handling of most contraband compounds, making this technique unacceptable for most field testing purposes, particularly the calibration of portal-type explosive detectors located in heavily populated areas. Even when designing and testing detectors in the laboratory, where using individuals highly trained in the handling of contraband is more practical, the frequency at which testing must be performed renders this approach undesirable.
Alternative calibration techniques rely on solutions prepared by dissolving known quantities of a particular contraband in a fixed amount of solvent to yield a solution having a known concentration of contraband. In one of these techniques, the detector is opened and the solution is applied directly to the collector train. This approach, however, may prove difficult and time-consuming for the lay operator. Further, while this technique may be helpful to determine how effectively the detector measures the quantity of substance disposed on the collector, it says little about either how well that substance is collected from the airstream, or whether the airstream is properly funnelled past the collector.
The contraband solution can instead be applied to a sorbent material, such as a paper towel or a gauze pad. When later sampled by a hand-held explosive detector or brought into a portal, these pads emit vapors and/or particles that replicate those discharged by an actual contraband sample. This technique, while relatively easy to perform, is generally only suitable for front-to-back qualitative analysis; that is, determining whether the detector properly indicates the presence of some critical amount of contraband. The concentration of vapors or particles is not known accurately enough to allow for reliable, repeatable quantitative sensitivity analysis.